1. Field of the Invention
The present invention generally relates to directional control valves of the type in which a flow control is incorporated into the control valve for controlling the rate of fluid flow through the control valve in proportion to selective positioning of the valve spool, and the flow rate is in proportion to the cross-sectional flow area of a fluid flow path that is established and sized by the valve spool positioning.
The present invention also generally relates to load responsive hydraulic systems in which the pump pressure and the effective output of the pump are controlled to maintain the system pressure at a predetermined pressure magnitude above the highest load actuating pressure that exists in a plurality of directional control valves. The control of the pressure and effective output of a variable displacement pump are controlled by control of the pump displacement; and the control of the pressure and effective output of a fixed displacement pump are controlled by a by-pass valve that discharges excess pump flow to a sump.
The present invention specifically relates to load responsive systems in which the system pressure is controlled to a predetermined pressure magnitude above the highest load actuating pressure of a plurality of directional control valves; and at least one directional control valve includes a flow control for reducing the system pressure to a predetermined pressure magnitude above the load actuating pressure of that one control valve; whereby fluid flow to the fluid motor connected to that control valve is substantially proportional to valve spool positioning irrespective of the system pressure.
The present invention more specifically relates to control valves in which flow control means is provided for achieving proportional control of fluid flow both to and from a fluid motor.
2. Description of the Prior Art
Load Responsive Systems
Early load responsive systems were disclosed by Allen, U.S. Pat. No. 2,892,312, in which the displacement of a variable displacement pump was controlled by a highest load actuating pressure, and Lee, U.S. Pat. No. 3,145,734, in which excess fluid output of a fixed displacement pump was by-passed to a sump to maintain the system pressure at a predetermined pressure magnitude above a load actuating pressure.
A more recent load responsive system, and one that may utilize either variable or fixed displacement pumps, is taught by McMillen et al., U.S. Pat. No. 3,693,506.